1. Field of the Invention:
The present invention relates to a catalytic composite for purifying exhaust gases and a method for preparing the catalytic composite. More particularly, it relates to a catalytic composite capable of completely burning unburnt portions of hydrocarbons and carbon monoxides released from various kinds of combustion equipment so as to decompose them into carbon dioxide and water.
2. Description of the Prior Art:
As an oxidation catalytic composite which converts hydrocarbon and carbon monoxide into carbon dioxide and steam in the coexistence of air, there is, for example, a catalytic composite including a catalyst made of a metal of the platinum group such as platinum, palladium or rhodium and also including a heat resistant material made of alumina, silica, or the like on which the catalyst is supported. It is known that this catalytic composite has a high degree of oxidation activity. Thus, it is widely used as a catalytic composite for purifying exhaust gases.
Base metals such as cobalt, nickel and iron are also used as a catalyst in the catalytic composite for purifying exhaust gases. In recent years, however, research has been conducted to develop catalysts made of compound oxides including a plurality of base metals, rather than catalysts of metal oxides including a single base metal. Among the compound oxides which have been developed as the catalyst, compound oxides having perovskite structures of the formula RMO.sub.3 have attracted attention because they have a high degree of oxidation activity. Among the perovskite-type compound oxides, LaCoO.sub.3 and LaMnO.sub.3 are particularly excellent in oxidation activity. It has been reported that the oxidation activity of these perovskite-type compound oxides are further improved by replacing some of the metals at the R-sites in the compound oxides with divalent alkaline earth metals (e.g., strontium).
In general, a catalytic compound for purifying exhaust gases is used for both high-temperature combustion equipment and low-temperature combustion equipment. The temperature of the exhaust gases released from the former ranges from 700.degree. to 800.degree. C. while the temperature of those released from the latter ranges from 300.degree. to 400.degree. C. Therefore, a catalytic compound for purifying exhaust gases is required to have excellent heat resistance at a high temperature of 700.degree. to 800.degree. C. and excellent oxidation activity at a low temperature of 300.degree. to 400.degree. C. Catalysts made of perovskite-type compound oxides including base metals are generally superior in heat resistance at a temperature of 700.degree. to 800.degree. C., but inferior in oxidation activity at low temperatures, as compared with catalysts made of a metal of the platinum group. In order to improve the low-temperature oxidation activity, there has been proposed the addition of a small amount of metal from the platinum group to the perovskite-type compound oxides. By the addition of a small amount of metal of the platinum group, the low-temperature oxidation activity of the resulting perovskite-type compound oxide can be improved. However, when this compound oxide is heated to a temperature of 700.degree. to 800.degree. C., the metal of platinum group added thereto deteriorates due to the high temperature, thereby reducing the degree of oxidation activity thereof.